One does not need to be in the new covenant to get their sins forgiven.
The new covenant is between Jehovah God and 144,000 Israelites of the spirit as they are kings and priest.
Any non-Israelites of the spirit who accept Jesus are saved by the high priest Jesus with his sacrifice.
Do you need a specific scripture telling you that? That's the problem with JWs is that they use scriptures and build on them and then build further upon them. But writers of scriptures don't write for all questions especially ones not asked of them.
Such things were taught by the Law and the temple, not specific scripture. Scriptures are interpretation of the law and temple.